Wireless communications system refers generally to any telecommunications system which enable a wireless communication between the users and the network. In mobile communications systems users are capable of moving within the service area of the system. A typical mobile communications system is a Public Land Mobile Network (PLMN).
At present third generation mobile systems, such as Universal Mobile Communication System (UMTS) and Future Public Land Mobile Telecommunication System (FPLMTS) later renamed as IMT-2000 (International Mobile Telecommunication 2000), are being developed. The UMTS is being standardized in ETSI (European Telecommunication Standards Institute) whereas ITU (International Telecommunication Union) is defining the IMT-2000 system. The radio interface is likely to be based on a wideband CDMA (code division multiple access), and therefore the third generation systems are often referred to as Wideband CDMA systems (WCDMA). These future systems are basically very much alike.
FIG. 1 shows a simplified UMTS architecture with the external reference points and interfaces to the UMTS Terrestrial Radio Access Network, UTRAN. The UTRAN consists of a set of Radio Access Networks RAN (also called Radio Network Subsystem RNS) connected to the Core Network CN through the interface Iu. These Radio Network Subsystems can be interconnected through the interconnection point (reference point) Iur. The interfaces Iu(s) and Iur are logical interfaces. Iur can be conveyed over physical direct connection between RANs or via any suitable transport network. Each RAN is responsible for the resources of its set of cells. For each connection between a mobile station MS and the UTRAN, one RAN is the Serving RAN. A RAN consists of a Radio Network Controller RNC and a multiplicity of base stations BS. The RNC is responsible for he handover decisions that require signalling to the MS. The base stations are connected to the RNC through the Iub interface. The core network CN is a conventional or future telecommunication network modified to efficiently utilize the UTRAN in a wireless communication. Telecommunication networks that are thought to be suitable core networks are second generation mobile communication systems (PSTN), such as GSM, ISDN (Integrated Services Digital Network), B-ISDN (Broadband ISDN), PDN (Packet Data Network), ATM etc.
FIG. 2 gives an overview of the assumed protocol environment in the third generation systems. Categorically, we can find three layers of the ISO/OSI layer model (International Standards Organisation/Open Systems Interconnection): physical layer (Layer 1, L1), data link layer (Layer 2, L2), and network layer (Layer 3, L3). In FIG. 2 the Layer L3 includes Radio Resources Control (RRC) protocol and upper user plane protocols. RRC takes care of all radio resources management. It negotiates quality of service QoS for a bearer service and on the basis thereof chooses needed transport format(s), (bitrates, type of coding, physical layer multiplexing), performs allocations (codes etc.), allocates identifiers for MS:s and bearer services, signals all of these parameters to MS, and supervises all handovers. User plane protocols relate to any upper layer transmission and signalling protocols. As used herein the term L3 protocols may also include the Link Access Protocol LAC set up between the MS and the core network CN although LAC may also be said to be an L2 protocol, LAN provides a peer-to-peer transportation of user data.
Layer L2 functions include the Radio Link Control (RLC) protocol and the Medium Access Control MAC. The RLC provides a radio-solution-dependent reliable link over the radio path. It takes care of segmentation and assembly of the Layer 3 data before and after transmission over the radio path, respectively, as well as retransmissions. Under the RLC the MAC function controls the mapping of the RLC protocol data units (RLC PDUs) into physical channels in the physical layer. The physical layer includes all the schemes and mechanisms used to make communications possible on the radio channel. These mechanisms include, for example, modulation, power control, coding and timing.
The RLC is capable of segmenting the higher layer PDUs. The segmenting allows a larger higher layer (e.g. L3, LAC) data unit to be split into smaller units (segments) on the lower layer (RLC). When segmenting is used, the transmitting end should indicate to the receiving end whether the same higher layer unit will continue in the next lower layer unit or a new higher level unit one will be started in the next lower layer unit. This information is needed in the receiver (either the mobile station (MS) or the network (NW)) to correctly assemble the segmented data.
In a prior art approach, a separate indicator has been used in each lower layer data segment to specify, whether the higher layer unit starts, ends or continues in the present data segment. Possible values may be the following, for example: 11 start & end; 10 start & continue; 00 continue; and 01 continue to end. The disadvantage of the prior art approach is that this extra field uses extra space in the protocol signalling and thereby causes extra overhead.